I post this here as a possible solution for those who are experiencing a similar behavior.
I migrated a bunch of ESP8266/ESP8285 devices from Tasmota to ESPHome, but all disconnected from HA Core every couple of minutes or so.
I solved this issue on all those devices at once by migrating to ESPHome-dev (1.15.0-dev).
Just to give an idea, this is what the history shows forone of those devices, a RGBWW bulb. The red line is the reboot following the migration to ESPHome 1.15.0-dev